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Help with Weight and Balance envelope C172


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Liam Sandie
Liam Sandie
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Hi all, just subscribed to SD now Android is supported (hope the full range of features make it across).

Anyway I am having real difficulty setting up the w&b envelope for our aircraft. There is already an online envelope which uses a Microsoft spreadsheet. This can be found here:-

http://www.dukeriesaviation.f9.co.uk/downloads/wb/G-ROLY_WeightBalance.xls

Can someone please assist me with using this graph to set up my Skydemon as I cannot get it to appear correct at all.

Many thanks

Liam

https://www.instagram.com/liam548/


stevelup
stevelup
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The reason you are struggling is because the graph in your spreadsheet shows the moment, not the CG.

I've tried converting the moment back (by dividing the moment by the weight), but it still doesn't make sense - I have no idea where the last figure came from...

I'd suggest carefully checking this against the POH instead.

Disregarding the last figure, the graph comes out OK and looks sane when compared with the numbers in a random POH I found on the internet, but you must do it properly using your figures.
stevelup
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Liam Sandie
Liam Sandie
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Thank you Steve. Is the link the link to the file for sd?

https://www.instagram.com/liam548/


Lyndon
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Think I've worked out the figures to input into SD, but the graph (arm length vs weight) comes out a little differently shaped to the one on the spreadsheet (moment vs weight).

instead of moment x 1000 weight of the POH

50.7 1500
67 1950
87.5 2450
107.3 2450
84.3 1500





I get this arm weight in sky demon

33.8 1500
34.36 1950
35.71 2450
43.8 2450
56.2 1500

I've tried a few plane configurations of various weights in different locations on both the spreadsheet and SD and they do appear to be working the same so I guess its right, its just a little disconcerting they look so different.

Can anyone check this for me please?

-Lyndon







Liam Sandie
Liam Sandie
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Steve what does your point mean regarding the last figure?

https://www.instagram.com/liam548/


stevelup
stevelup
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The moment graph will lean to the right, the CG graph would usually have a straight right hand edge..

Personally, I'd ignore the graph in your POH and just use the textual information.
Liam Sandie
Liam Sandie
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Sorry for mis understanding but ignore the graph? The issue I think is sd sets their graphs up different to most poh

https://www.instagram.com/liam548/


stevelup
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The Moment Graph in your POH is there to help you do the calculation manually. That's why it is a Moment graph, and not a CG graph. You can easily tell one from the other - a moment graph will lean extensively to the right, a CG graph will almost always have a square right hand edge.

Have a look at this POH (Link)

On Page 6-11 is a Moment graph (this is what you are trying to use)
On Page 6-12 is a CG Limit graph (this is what SkyDemon needs)

It should be possible to convert a Moment graph into a CG graph by simply dividing the moment by the weight.

However, when I do that with your figures, it doesn't work (the right hand edge ends up sloping to the right by a considerable amount). That suggests that your starting figures are wrong.


Moment/1000

Moment

Weight

Arm

50.70

50700

1500

33.80

67.00

67000

1950

34.36

87.50

87500

2450

35.71

107.30

107300

2450

43.80

84.30

84300

1500

56.20


(The figures in bold should be the same)



If I do it to the figures in the moment graph in the POH listed above, it ends up with a square right edge as expected.


Moment/1000

Moment

Weight

Arm

52.50

52500

1500

35.00

68.00

68000

1950

34.87

88.50

88500

2300

38.48

109.00

109000

2300

47.39

71.00

71000

1500

47.33


Instead of trying to interpret the graphs, it is far simpler and more accurate to use the figures from page 2-7 of the book:-


'Center of Gravity Range'

Forward 35 inches aft of datum at 1950lbs with straight line variation to 38.5inches aft of datum at 2300, Aft 47.3


So from that, we can simply create...


35,1500, 35,1950, 38.5,2300, 47.3,2300, 47.3,1500


...which we know is right, and didn't rely on squinting at a graph.


Edited 5/23/2013 3:37:00 PM by stevelup
Liam Sandie
Liam Sandie
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I see. Thank you very much for your help Smile

https://www.instagram.com/liam548/


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